Zein is a group of plant proteins that can be extracted from corn or corn-protein-containing substrates, such as corn gluten meal and has utility as a raw material for a variety of non-toxic and renewable polymer applications. Zein is classified as GRAS (Generally Recognized As Safe) by the U.S. Food and Drug Administration and has a variety of commercial uses including the manufacture of edible food packaging, edible films, biodegradable plastic resins, chewing gum base, tablet-coating compounds, adhesives, coatings for paper cups, soda bottle cap linings, etc. Zein can also be processed into resins and other bioplastic polymers, which can be extruded or rolled into a variety of plastic products.
Zein belongs to a class of proteins called prolamins, which are soluble in alcohol, and comprises approximately forty to fifty percent of the total protein in corn, or about four percent of the corn kernel. Zein has been further divided into four subclasses: alpha-zein, beta-zein, gamma-zein, and delta-zein. Alpha-zein is the primary commercially used zein and accounts for about seventy percent of the zein in corn. Beta-zein accounts for about five percent of the zein in corn. Gamma-zein accounts for approximately twenty to twenty-five percent of the zein in corn and delta-zein accounts for about one to five percent of the zein in corn. Each zein type (alpha, beta, gamma, and delta) has a different amino-acid profile and exhibits slightly different physical properties.
Zein can be extracted and recovered from corn or co-products from corn processing. The composition and characteristics of the resulting zein may depend on the starting material and the extraction solvent.
For example, ethanol can be produced from grain-based feedstocks (such as corn). Ethanol production from corn produces fermentation products (e.g., co-products) that are suitable for use as starting materials for zein extraction. One such co-product includes corn gluten meal (CGM), a by-product of wet milling ethanol production. In a typical wet milling process, ethanol is produced from corn by first steeping the corn kernels in water that contains sulfur dioxide, and then separating the kernels into endosperm, fiber and germ. The endosperm is further processed to produce starch and corn gluten, which can be dried into Corn Gluten Meal. Corn Gluten Meal is a typical starting material for zein extraction due to its high protein content (sixty percent or greater). However, the sulfur dioxide or other chemicals that may be used during the preparation (e.g., in a steeping process) of corn gluten meal may adversely affect zein quality.
Another co-product from which zein may be extracted comes from a dry-milling process. In a typical dry-milling process, a starch containing material, such as corn, is ground into flour and slurried with water and enzymes. The slurry may be cooked to liquefy the starch and to facilitate saccharification. Additional enzymes may be added to complete saccharification to break down the starch into simple sugars (e.g. glucose) that can be fermented using an ethanologen (e.g. yeast). The fermentation produces a fermentation product that comprises a liquid portion or component and a solids portion or component. The liquid portion comprises ethanol, water, and other soluble components. The residual solids comprise, for example proteins, fiber, oil, and other insoluble components.
The fermentation product comprising a liquid component and a solids component may be distilled to separate ethanol and whole stillage (e.g. wet solids or fermentation solids). Whole stillage comprises residual solids and water, and may be further separated into wet cake and thin stillage. Wet cake (wet solids) can be dried into meal such as dried distillers grains (DDG); thin stillage can be reduced to syrup and added to the wet cake or meal during the drying process to produce dried distillers grains with solubles (DDGS). Meal such as DDG and DDGS can be used as an animal feed product. The wet cake or dried meals (DDG and/or DDGS) may be used as a starting material for zein extraction.
According to an alternative process, for example as described in U.S. Patent Application Publication No. 2005/0239181 (which is hereby incorporated by reference in its entirety), starch may be converted into sugars and fermented in a raw-starch process without “cooking” or liquefaction. Heat damage to proteins and other components of the slurry may be avoided by using the raw-starch process. The resulting wet cake, DDG or DDGS from the raw starch process may likewise be used for zein extraction.
Further, a dry fractionation process that does not utilize sulfur dioxide may be used instead of wet milling to fractionate the corn into endosperm, fiber and germ. The amount of residual solids in the fermentation product can be reduced by fractionation and by eliminating fiber and germ, both low in starch, from fermentation. Endosperm is primarily comprised of starch and protein with small amounts of fiber and oil present. Zein is also concentrated in the endosperm; more than half of the endosperm protein may be comprised of zein. When endosperm is fermented, the residual solids comprise a high concentration of zein. The dried residual solids from endosperm fermentation are high in protein and result in a meal that is called “high protein dried distiller grains” (DDG HP). DDG HP is well suited as a starting material for zein extraction.
As previously noted, four different types of zein proteins are present in corn: alpha-zein, beta-zein, gamma-zein, and delta-zein. Each zein type has a different amino-acid profile and exhibits slightly different properties. The predominant commercially available zein sold in industry currently is comprised almost entirely of alpha-zein. The melt strength of this zein is low making it ineffective as a material for blown films. Films of commercial zein are brittle and weak compared to synthetic films. These physical properties are the result of alpha-zein containing only one cysteine, thus making it able to only form one disulfide bond. In contrast, beta-zein and gamma-zein contain greater number of cysteine amino acid bases, at 12 and 15 cysteine respectively. Thus, zein formulations with greater concentrations of beta- and gamma-zein may have higher melting temperatures and greater plasticity. This may enable zein products to be utilized in a wider range of applications, including blown films, and plastics of greater strength.
It would be advantageous to provide for systems and methods for extracting protein from a fermentation product which yields high levels of beta- and gamma-zein. It would also be advantageous to provide for a system for producing ethanol that facilitated the recovery of co-products, including high cysteine zein compositions, extracted from components of the fermentation product. Further, it would also be advantageous to provide for a zein composition with high cysteine for improved physical properties.